Engine control systems require accurate control of exhaust gas recirculation (EGR) for controlling regulated emissions and achieving fuel economy improvements. One type of exhaust gas recirculation system externally recirculates the exhaust gas from the exhaust manifold to the intake manifold with a controllable valve placed in the flow path between the exhaust manifold and the intake manifold. Typically, the valve is pneumatically operated and controlled by an electronic engine controller.
One approach to controlling the valve is to create a signal based on a throttle position. In such a system, the throttle position is used as an input to two parallel filters. The first filter acts as a differentiator and the second filter acts as a fixed-rate tracking filter. As a result, the differentiation filter mostly controls turning the valve off, while the second filter mostly determines the time period for keeping the valve off, or closed. Such a system is described in U.S. Pat. No. 5,533,489.
Another approach to controlling exhaust gas recirculation flow is to use a feedback variable to assure that the actual exhaust gas recirculation flow converges to the desired exhaust gas recirculation flow. One method is to use a differential pressure measured across an orifice in the exhaust flow path. Then, the differential pressure can be used to infer the actual exhaust gas recirculation flow. Finally, an error between the actual and desire exhaust gas recirculation flow is used to create a control signal that is sent to the actuator. Thus, the system can compensate for the effects of engine and component aging, as well as other errors in the system. Such a system is disclosed in U.S. Pat. No. 5,190,017.
The inventors herein have recognized several disadvantages with the above systems. For example, the throttle position filters described above are for controlling a valve that is simply on or off. Also, the differentiation filter does not discriminate between opening and closing throttle position. This is an important characteristic because most exhaust gas recirculation valves have asymmetric behavior. Typically, the valve will close much faster than it opens.